Amyotrophic lateral sclerosis (ALS) is a fatal degenerative disease of motor neurons with no effective treatment. New treatments are desperately needed, but clinical trials are hampered by the heterogeneous nature of ALS, such as the variablity and unpredictability of disease progression. There is also a paucity of sensitive measures to track responses to treatment leading to lengthier and more costly trials and a greater burdern on the patient. Through an R21-funded RNA sequencing project on human ALS muscle (Si et al., 2014), we have identified fibroblast growth factor 23 (FGF23) as a promising biomarker in ALS that might help fill this clinical gap. In our preliminary data, we observe that plasma FGF23 levels increase with disease progression in the SOD1G93A mouse model of ALS, and are elevated in a small cohort of ALS patients where levels appear to correlate positively with rapid decline in functional rating scores. In Specfic Aim 1, we hypothesize that plasma FGF23 levels will track disease progression and that higher FGF23 levels will predict a poorer prognosis in ALS patients. This hypothesis will be tested by measuring plasma FGF23 by ELISA in a large cohort of well characterized and longitudinally followed ALS patients from a national biorepository. The cohort will represent the diversity of clinical phenotypes inherent in this disease, including fast- and slow-progressors. In our preliminary studies, we observed also that FGF23 is elevated in muscle, spinal cord and cortex of the SOD1G93A mouse, suggesting that FGF23 signaling is activated in ALS. FGF23 acts as a pro-inflammatory cytokine, can be produced by macrophages, and can activate macrophages and other immune cells to produce inflammatory cytokines such as TNF-? and IL-1?. Since neuroinflammation accelerates disease progression in ALS, elevated FGF23 may represent a disease modifier that contributes to the clinical heterogeneity of ALS. In Specific Aim 2, we hypothesize that FGF23 is produced in the CNS of the SOD1G93A mouse by glia or invading immune cells, and that it can activate these cells to produce inflammatory cytokines. We will perform immunohistochemistry to localize FGF23 in CNS and muscle tissues of the SOD1G93A mouse and with human ALS tissues from the PI's tissue repository. We will stimulate microglia and astroglia from wild-type and SOD1G93A mice with FGF23 and assess activation, including the induction of inflammatory cytokines and chemotaxis. This aim represents an initial investigation into the potential role of FGF23 in the pathophysiology of ALS and will pave the way for future mechanistic studies. In summary, this bench-to-bedside proposal represents a novel direction in ALS with compelling translational implications. It vertically integrates basic and clinical science approaches, capitalizing on the collaborative spirit at UAB and the U. of Miami, to address a major gap in testing new treatments in ALS. At the same time, it will begin to investigate a new signaling pathway that might provide insight into molecular mechanisms that drive clinical progression in ALS.